A joint, such as the ankle, knee, hip or shoulder, generally consists of two or more relatively rigid bony structures that maintain a relationship with each other. In the case of the spine, a motion segment generally consists of two vertebral bodies, a disc and two facet joints. Soft tissue structures spanning the bony structures hold the bony structures together and aid in defining the motion of one bony structure relative to the other. In the knee, for example, the bony structures are the femur, tibia and patella. Soft tissue structures spanning the knee joint, such as muscles, ligaments, tendons, menisci, and capsule, provide force, support and stability to facilitate motion of the knee. Muscle and tendon structures spanning the knee joint, as in other joints of the body and in the spine provide dynamics to move the joint in a controlled manner while stabilizing the joint to function in an orderly fashion. The joint is dynamically stabilized by contraction of primary muscles to move the joint in a desired direction combined with antagonistic muscle contraction to direct resultant joint loads within favorable orientation limits relative to the bony structures of the joint. It is believed that proprioceptive feedback provides some of the control or balance between primary and antagonistic muscle contraction.
In an articulating joint, a smooth and resilient surface consisting of articular cartilage covers the bony structures. In the spine, the disc, consisting of an annulus and a nucleus, spans the space between adjacent vertebral bodies and two facet joints provide articulation posteriorly. The articular surfaces of the bony structures work in concert with the soft tissue structures spanning the joint to form a mechanism that defines the envelop of motion between the structures. Within a typical envelop of motion, the bony structures move in a predetermined pattern with respect to one another. When articulated to the limits of soft tissue constraint, the motion defines a total envelop of motion between the bony structures. In the knee, the soft tissue structures spanning the joint tend to stabilize the knee from excessive translation in the joint plane of the tibiofemoral compartments. Such tibiofemoral stability enables the femur and tibia to slide and rotate on one another in an orderly fashion. The motion of the patella relative to the femur in the patellofemoral compartment is related to tibiofemoral motion because the patella is linked at a fixed distance from the tibia by the patellar ligament.
Current methods of preparing a joint to receive implants that replace the articular surfaces or motion segments involve an extensive surgical exposure. In traditional total knee arthroplasty, the surgical exposure, ligament release and sacrifice of the anterior cruciate ligament must be sufficient to permit the introduction of guides that are placed on, in, or attach to the femur, tibia or patella, along with cutting blocks to guide the use of saws, burrs and other milling devices, and other instruments for cutting or removing cartilage and bone to provide a support surface for implants that replace the artificial surfaces or motion segment. For traditional knee joint replacement, the distal end of the femur may be sculpted to have flat anterior and posterior surfaces generally parallel to the length of the femur, a flat end surface normal to the anterior and posterior surfaces, and angled flat surfaces joining the above mentioned surfaces, all for the purpose of receiving a prosthetic device. In general these are referred to as the anterior, posterior, distal and chamfer cuts, respectively. In current total knee arthroplasty proper knee alignment is attained by preoperative planning and x-ray templating. Anterior-posterior (A/P) and lateral x-ray views are taken of the knee in full extension. The mechanical axis of the tibia and of the femur is marked on the A/P x-ray. The angle between these lines is the angle of varus/valgus deformity to be corrected. In the A/P view, the angle of the distal femoral resection relative to the femoral mechanical axis, hence the angle of the femoral implant, is predetermined per the surgical technique for a given implant system. Similarly, the angle of the tibial resection relative to the tibial mechanical axis, hence the angle of the tibial implant, is predetermined per the surgical technique for a given implant system. The femoral resection guides are aligned on the femur to position the distal femoral resection relative to the femoral mechanical axis and the tibial resection guides are aligned on the tibia to position the proximal tibial resection relative to the tibial mechanical axis. If the cuts are made accurately, the femoral mechanical axis and the tibial mechanical axis will align in the A/P view. Once the femur and tibia have been resected, the medial and lateral collateral ligaments may be released to balance the knee. Soft tissue balancing is generally done with the knee in full extension. The spacing between the femur and tibia at full extension is used to guide ligament release to attain an appropriate extension gap.
Typically, an appropriate extension gap is evidenced by parallel orientation of the distal femoral resection to the tibial plateau resection and with a gap sufficient to accommodate the femoral and tibial implants. This approach addresses knee alignment and balancing at full extension. Knee alignment and tissue balance at 90.degree. of flexion is generally left to surgeon judgment and knee alignment and tissue balance throughout the range of motion has not been addressed in the past. In aligning the knee at 90.degree. the surgeon rotates the femoral component about the femoral mechanical axis to a position believed to provide proper tensioning of the ligaments spanning the knee.
Current implants and instruments for joint replacement surgery have numerous limitations. These relate to the invasiveness of the procedure and achieving proper alignment, soft tissue balance and kinematics of the joint with the surgical procedure. Such difficulties are present in all joint replacement surgery. Although the spinal disc is not an articular joint, interest in restoring the kinematic function of a degenerated disc has lead to spinal arthroplasty incorporating metal and/or plastic articulating surfaces. Polymers, including hydrogels and urethanes, have also been used to restore spinal disc function. Such spinal implants are preferably placed via minimally invasive surgical approaches and restore motion and kinematics, hence require accurate alignment and orientation of the implant components one to another. In addition, the kinematics of a spinal motion segment are defined by the combined motion across the disc which is a function of the annulus, nucleus, anterior ligament, posterior ligament, facet joint articulation and muscles spanning the motion segment. A spinal motion segment is the motion between adjacent vertebral bodies.
A difficulty with implanting modular knee implants in which the femur or tibia is resurfaced with multiple components has been achieving a correct relationship between the components. For ease of description, multiple components comprising a component such as a femoral component will be referred to as subcomponents. For example, a modular femoral component may include subcomponents for the trochlea, the lateral femoral condyle and the medial condyle, and reference to a “femoral component” includes subcomponents in the case of a multi-piece femoral component.
In the case of a plurality of subcomponents resurfacing the distal femur or proximal tibia, the orientation and alignment of the subcomponents to each other has largely not been addressed. This may account for the high failure rates in the surgical application of free standing compartmental replacements used individually or in combination. Such compartmental replacements include medial tibiofemoral compartment, lateral tibiofemoral compartment, patellofemoral compartment and combinations thereof. Component malalignment may account for the higher failure rate of uni-compartmental implants relative to total knee implants as demonstrated in some clinical studies. When considering bi-compartmental and tri-compartmental designs, orientation and alignment of subcomponents, as well as components, is critical to avoid accelerated wear with a mal-articulation of the implant.
Surgical instruments available to date have not provided trouble free use in implanting multi-part implants wherein the distal femur, proximal tibia and posterior patella are prepared for precise subcomponent-to-subcomponent and component-to-component orientation and alignment. While current femoral alignment guides aid in orienting femoral resections relative to the femur and current tibial alignment guides aid in orienting tibial resections relative to the tibia, they provide limited positioning or guidance relevant to correct subcomponent-to-subcomponent alignment or orientation. Nor do such alignment guides provide guidance relevant to soft tissue balance (i.e. ligament tension to restore soft tissue balance). Moreover, they provide limited positioning or guidance relevant to correct flexion/extension orientation of the femoral component, to correct axial rotation of the femoral component, nor to correct posterior slope of the tibial component. For the patellofemoral joint, proper tibiofemoral alignment is required to re-establish proper tracking of the patella as defined by the lateral pull of the quadriceps mechanism, the articular surface of the femoral patellar groove and maintaining the tibiofemoral joint line. For optimum knee kinematics, femoral component flexion/extension and external rotation orientation, tibial component posterior slope and ligaments spanning the joint work in concert maintaining soft tissue balance throughout the knee's range of motion.
For patients who require articular surface replacement, including patients whose joints are not so damaged or diseased as to require whole joint replacement, the implant systems available for the knee have unitary tri-compartmental femoral components, unitary tibial components, unitary patellar components and instrumentation that require extensive surgical exposure to perform the procedure.
It would be desirable to provide surgical methods and apparatuses that may be employed to gain surgical access to articulating joint surfaces, to appropriately prepare the bony structures, to provide artificial, e.g., metal, plastic, ceramic, or other suitable material for an articular bearing surface, and to close the surgical site, all without substantial damage or trauma to associated muscles, ligaments or tendons, and without extensive distraction of the joint. To attain this goal, implants and instruments are required to provide a system and method to enable articulating surfaces of the joints to be appropriately sculpted using less or minimally invasive apparatuses and procedures, and to replace the articular surfaces with implants suitable for insertion through small incisions, assembly within the confines of the joint cavity and conforming to prepared bone support surfaces.